1. Field of the Invention
The present invention relates to a sputtering apparatus, and more particularly, to a sputtering apparatus having a cathode capable of depositing a homogeneous thin film of a uniform thickness on the surface of a relatively large substrate while effectively consuming the entirety of a target surface.
2. Description of the Related Art
In conventional sputtering apparatus, various types of cathode structures have been proposed. Among them, the magnetron cathode type is most widely applied to industrial fabrication processes because of its high deposition rate. Various types of magnetron cathode types are known. Such magnetron cathode types are described in, for example, "Thin Film Process" (published by Academic Press in 1978 and edited by J. L. Vossen and W. Kern from pages 75 to 113) or in "Thin Film Handbook" (published in 1983 and edited by Nippon Gakujutsu Shinkokai Thin Film 131st Committee page 186 to 189).
Of the magnetron cathode types, the planar magnetron cathode provided with a planar target is currently the most advantageous from the viewpoint of industrial fabrication.
In conventional sputtering apparatus to which a planar magnetron cathode having a rectangular planar shape is applied, the following problems (1) through (4) arise with deposition of a thin film on a large area substrate:
(1) Increase in the size of the apparatus PA1 (2) Waste of target due to non-uniform erosion thereof PA1 (3) Heterogeneity of the film deposited on a substrate due to non-uniform ion bombardment and non-uniformity of plasma space density distribution above the surface of the target PA1 (4) Generation of dust particles due to non-uniform ion bombardment of the surface of the target PA1 (1) Increase in the size of the apparatus: PA1 (2) Waste of target due to non-uniform erosion thereof; PA1 (3) Heterogeneity of the deposited film on the substrate: PA1 (4) Generation of dust particles:
Each of the above problems (1) through (4) will be described in detail below.
The relative spatial relationship between the substrate and the target in the sputtering apparatus has been described in, for example, "Dry Process Application Technology" (written by Haruhiro Kobayashi, Takashi Okada and Naokichi Hosokawa and published by Nikkan Kogyo Shinbun Shuppan in 1984), pages 63 to 64. Normally, a thin film is deposited on a large substrate by the "substrate transferring" method. The "substrate transferring" method is a method of depositing a thin film on the surface of a substrate by sputtering in a sputtering apparatus in which a rectangular target is disposed while transferring (i.e. moving) the substrate along a plane parallel to the surface of the rectangular target. Japanese Patent Publication No. 63-65754 (Japanese Patent Laid-Open No. 60-86272) discloses a substrate transferring type sputtering apparatus in which a rectangular magnetron cathode is incorporated. In this substrate transferring type sputtering apparatus, the direction of substrate movement is parallel to the shorter side of the rectangular target in order to deposit a thin film having a uniform thickness.
When the dimensions of the substrate are larger than the length of the shorter side of the target, the deposition of a thin film having a uniform thickness on the substrate generally requires moving the substrate a sufficiently long distance during film deposition. This requirement, concomitant with a requirement for increase in productivity, increases the scale of the substrate transferring type sputtering apparatus for larger substrates.
In, for example, a sputtering apparatus marketed by the ANELVA Corporation, a magnetron cathode having a planar target whose shorter and longer sides are respectively 250 mm and 864 mm in length and a tray having a height of 950 mm and a width of 830 mm are employed to transport a glass substrate having a height of 700 mm and a width of 680 mm, to deposit a thin film having a uniform thickness within a tolerance of .+-.10% (ANELVA "3960S").
This sputtering apparatus further requires (i) a load-lock mechanism for transporting from the atmospheric side an unprocessed substrate into a deposition chamber where the sputtering process is performed and for transporting the processed substrate back out to the atmospheric side from the deposition chamber while maintaining and controlling the deposition chamber under a vacuum, (ii) a heating zone for heating the substrate prior to the deposition and (iii) a buffering space required to convey trays substantially successively, and thus has a height of about 2.5 m, a width of about 2 m and an overall length of 10 to 20 m. Finally, this is a huge system.
As has been described in the aforementioned literature, in the magnetron cathode, since the target surface thereof is subjected to non-uniform ion bombardment, it is eroded non-uniformly. This non-uniform erosion occurs due to utilization of a non-uniform magnetic field generated around the target surface in the magnetron cathode.
For example, in both the conventional cylindrical coaxial magnetron cathode and conventional circular planar magnetron cathode, a structure for displacing a magnet assembly provided within the cathode is provided to improve target waste and film thickness distribution caused by non-uniform erosion of the target (Japanese Patent Publication No. 55-27627 and Japanese Patent Laid-Open No. 3-6372). This structure is suitable for an electrode having a rotationally symmetrical structure. According to this structure, it is possible to make erosion across the target uniform in a circular planar magnetron cathode by rotating the magnet assembly around an axis. However, this structure is not effective as a means for making erosion across a planar rectangular target uniform.
Non-uniform ion bombardment on the target surface of the magnetron cathode also causes heterogeneity of the film deposited over the whole substrate. This influence of the non-uniform ion bombardment will be described below with reference to the structure described in the aforementioned Japanese Patent Publication No. 63-65754 (Japanese Patent Laid-Open No. 60-86272). This structure provides for the deposition of a thin film by a combination of a rectangular magnetron cathode and a substrate transferring type sputtering apparatus.
When a magnetron plasma is generated in a vacuum by applying a voltage between the cathode and the wall of a vacuum vessel acting as an anode to deposit a thin film by sputtering, electrons perform drift-motion along a closed tunnel path generated by the lines of magnetic force over the surface of the planar target of the rectangular magnetron cathode and make a looped locus. Since the drift electrons collide with the gas molecules to create ions and additional electrons, a plasma of a high density is generated along the locus of the drift electrons. The portion of the target surface located right below the locus of the drift electrons is subjected to strong ion bombardment and is thereby sputtered. In an actual apparatus, since a very large number of drift electrons are present, the locus thereof has a belt-like shape having a certain width. Thus, the plasma generated along this belt also becomes belt-like. The region on the target surface which is sputtered, called an erosion region, is also a loop region having a belt-like shape.
The substrate placed on a conveyed tray passes through the space in front of the target during the film deposition. At that time, a certain portion on the substrate passes across the space located above the two belt-like erosion regions which run parallel to the longer side of the rectangular target. The space located above these two erosion regions has a high plasma density. As is well known, particles (atoms) which are sputtered are ejected in all directions as well as in the direction perpendicular to the target surface. Thus, at certain positions (i.e. portions) of the substrate, accumulation of the sputtered particles (atoms) starts before the portion arrives at the belt-like erosion regions. The deposition rate increases as the portion on the substrate approaches the space above the first belt-like erosion region. The deposition rate is at a maximum right above the first belt-like erosion region. Thereafter, the deposition rate on the portion on the substrate temporarily decreases as the tray moves away from the first belt-like erosion region. The deposition rate increases again as the portion on the substrate approaches the space above the second belt-like erosion region, and it achieves the maximum deposition rate again when the portion on the substrate is directly right above the second belt-like erosion region. This is the second time the maximum deposition rate occurs. During subsequent conveyance of the tray, the deposition rate on the portion on the substrate gradually decreases as the substrate moves away from the space in front of the target.
Thus, the film deposition rate of the certain portion of the substrate varies with time. Also, the angle of incidence of the sputtered atoms accumulated on the surface of the substrate and the plasma density in the space above the surface of the substrate respectively vary greatly with time. These factors result in changes of the physical characteristics of the final thin film formed at each position of the substrate surface in the direction perpendicular to the film surface.
Non-uniformity of ion bombardment on the target surface of the magnetron cathode causes generation of dust particles during the film deposition process. As mentioned above, on the target surface of the rectangular magnetron cathode, a single closed belt-like erosion region, which is narrow as compared with the width of the target, is subjected to ion bombardment and is eroded. The other regions of the target surface are not eroded: rather, atoms ejected from the target and scattered by collision with the gas molecules are deposited thereon. After sputtering continues for a long time, sputtered particles (atoms) are deposited on that portion of the target surface other than the belt-like erosion regions. The film deposited on the target begins to peel off the target surface due to the internal stress of the film as the film thickness increases and bits of the peeled off film become dust particles. In electronic devices manufactured by processing fine patterns of the thin film on the substrate, generation of dust particles is regarded as one of the reasons for the increase of defects in products. Hence, development of a magnetron cathode electrode which can reduce the generation of dust particles has been desired.